Method for troubleshooting gas-lift wells

ABSTRACT

A method is provided for troubleshooting gas-lift wells, to identify whether gas-lift valves on the production tubing are open or closed, without the use of wireline tools. The method may also be used to detect leaks in the production tubing or in the well casing. A quantity of a tracer gas is injected into the lift-gas at the wellhead, and its return in fluid produced from the well is monitored as a function of time. The tracer&#39;s return pattern may be correlated with the depth of entry points and volumes of lift-gas entering along the length of the production tubing.

This is a division of application Ser. No. 323,600, filed Mar. 14, 1989,now U.S. Pat. No. 4,972,704.

BACKGROUND OF THE INVENTION

This invention concerns the field of production of oil and otherreservoir fluids from subterranean reservoirs, and particularly the useof gas-lift well systems. The invention provides a new method fortrouble-shooting gas-lift wells. It allows detection of leaks inproduction tubing and casing without the use of wireline tools, and itis particularly useful for determining whether the valves in a gas-liftwell are operating properly.

Prior methods of detecting leaks in wells have involved the use ofwireline tools. For example, U.S. Pat. No. 2,383,455 discloses a methodfor detecting casing leaks in a subterranean well by measuring thetemperature gradient of the well. A thermal recorder is lowered throughthe production tubing in the well to measure the temperature gradient ofthe well. A temperature anomaly at a given depth is indicative of a leakat that depth. It is also known in the art to use other wireline tools,such as mechanical calipers, sonic, or noise detection tools, to detecttubing and casing leaks in wells.

Such wireline tool methods have several disadvantages for the welloperator. The methods are usually performed by a well service company ona contract basis, which involves additional operating costs and losttime during well evaluation. It is usually necessary to shut downoperations on the well, at least during insertion and removal of thetools. Specialty tools are used, and interpretation of the results oftenrequires expert analysis. There is also some risk associated withinserting tools into a well, since if the tool is irretrievably lost inthe well, it may be necessary to implement expensive remedialoperations, or abandon the well. Consequently, the expense and riskassociated with using these wireline tool methods are significantdisadvantages for the well operator.

A method which does not involve the use of wireline tools has beendeveloped for detecting casing leaks in underground storage caverns.U.S. Pat. No. 4,474,053 discloses a method for detecting casing leaks inan underground cavern used to store hydrocarbons. An inert gas ismaintained under pressure in the annulus between two casings, and thepressure of the inert gas is continuously monitored at the wellhead. Adecrease in pressure of the inert gas at the wellhead is indicative of aleak in the casing.

In the subject invention, a new method is disclosed that allowsdetection of production tubing and casing leaks in gas-lift wells,without the use of wireline tools. The method may also be used to detectwhether gas-lift valves on the production tubing in a gas-lift well areopen or closed. This new method eliminates the cost and risk associatedwith the use of wireline tools. The method is also simple, and easy forfield personnel to perform with minimal equipment. The results obtainedare easy to interpret, allowing operating personnel to troubleshootwells at the well site and without the need to consult off-site experts.

SUMMARY OF THE INVENTION

The subject invention provides a simple, inexpensive method fortroubleshooting a gas-lift well system. The method is performed whilethe well is in operation, and without the use of wireline tools. Themethod may be used to determine whether gas-lift valves on a productiontubing string are open or closed, and to identify whether there areleaks in the production tubing or casing.

Application of the method requires the use of an injected fluid, whichis injected into the lift gas at the well in a quantity that willprovide a detectable amount of a tracer in a produced fluid recoveredfrom the well. The injected fluid is and/or generates one or moretracers in one or more of the reservoir fluids produced from the well.The presence of the tracer is detected in the produced fluid as afunction of time. As an alternative, a reduction in the proportion ofone or more components present in a produced fluid, occurring due to thepresence of a tracer in the produced fluid, is monitored as a functionof time. This information may be used to determine the point or pointsof entry of the injected fluid into the production tubing, therebydetermining the point or points of entry of the lift-gas into theproduction tubing. Consequently, it is possible to determine which ofseveral gas-lift valves are open or closed on the production tubing, andwhether there are any other openings, such as leaks, in the productiontubing. By monitoring the proportion of tracer present in the producedfluid as a function of time, it is possible to quantify the amount oflift-gas entering the production tubing at each point of entry. Throughmaterial balance calculations on the tracer, to determine whether anytracer is lost from the casing, the method may also be useful fordetecting the presence (or absence) of casing leaks.

Other purposes, distinctions over the art, advantages and features ofthe invention will be apparent to one skilled in the art upon review ofthe following.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a gas-lift well system.

FIG. 2 shows the concentration of tracer in gas produced from a gas-liftwell, resulting from application of the method of the subject invention.

DETAILED DESCRIPTION OF THE INVENTION

Gas-lift is an important method for artifically lifting reservoir fluidsfrom a subterranean reservoir to the surface when natural forces areinsufficient. It is widely used, primarily for oil wells andparticularly in offshore wells, but also occasionally for water wells. Asource of high pressure gas must be available. Usually, gas producedwith the oil is treated and compressed in a field compressor for use asthe lift-gas. In a gas-lift system, gas is injected into the annulusbetween the casing and the production tubing, and enters the productiontubing through a gas-lift valve, causing an increase in the gas-liquidratio inside the tubing above that gas-lift valve. Multiple gas-liftvalves are usually included in the production tubing at multiple depthsto unload the well initially and to establish the initial point of gasinjection, which is usually above the total depth of the well.

During initial start-up of the well, the gas-lift valves are initiallyopen. As fluid in the casing and production tubing is lifted from thewell, pressure decreases in the annulus between the casing and theproduction tubing. The gas-lift valves in the production tubing havepressure settings that decrease with depth. Consequently, the valvesclose in sequence from the top of the well to the bottom of the well,until only the bottom gas-lift valve is left open, and production fromthe bottom gas-lift valve is established. Most well designs call foronly one gas-lift valve to be open once continuous production isobtained. Since reservoir pressures generally decline over time, it isusually necessary to inject lift gas deeper in the well over the life ofa project. Consequently, the production tubing may initially contain"dummy" gas-lift valves at the lowest points on the tubing that may bereplaced, when necessary, with active gas-lift valves. The lower activegas-lift valve would then be used as a new, lower point for continuousgas injection.

In a gas-lift well, it is necessary to periodically check to see if thegas-lift system is operating properly. Once the well is onstream, onlythe bottom gas-lift valve should be open. Occasionally, a gas-lift valveabove the bottom gas-lift valve will remain open. The lift-gas enteringthe production tubing from that valve is essentially wasted, since thelift-gas entering at the bottom of the well is sufficient to provideproduction at the well. Also, the bottom gas-lift valve may occasionallyfail closed, resulting in inefficient operation of the gas-lift system.In addition, leaks in the production tubing may also serve as entrypoints for lift-gas. If excess lift-gas enters the production tubing, itwill decrease the production of oil from the well and increase operatingcosts due to the wasted excess gas.

The present invention provides an inexpensive, low-risk method fordetermining whether gas-lift valves along the production tubing are openor closed. The method may also be used to detect leaks in the productiontubing or casing. No wireline tools are required, and the method may beapplied to a well while it is in operation. The results obtained areeasy to interpret, allowing operating personnel to troubleshoot wells atthe well site, and without the need to consult experts away from thewell.

A key advantage of the invention is its simplicity. Where analyticalequipment is used to detect or monitor and to automatically record thepresence and/or proportion of a tracer in a produced fluid, theequipment may be set up by operating personnel who may then proceed todo other tasks until the recorded data are available for analysis. Forexample, where a graphic representation of the concentration of tracerpresent in a produced fluid as a function of time is obtained, it isvery simple for the operating personnel to identify the number of entrypoints on the production tubing, and the relative proportions oflift-gas entering the production tubing at each point. In a well that isoperating properly, only one, well defined peak will occur at anexpected response time, representing one open gas-lift valve at theproper depth on the production tubing. After operating personnel becomefamiliar with using the method, it may be possible for such personnel tosimply look at a graphical representation of the data obtained fromapplication of the subject invention, and determine whether a gas-liftwell is operating properly. Since wells in a given field are oftensimilarly designed, the same pattern may be exhibited by most of thewells in a field.

The following is a detailed description of an apparatus used in themethod of the subject invention, with specific reference to FIG. 1. Alift-gas supply source 1 supplies lift-gas through a lift-gas supplyline 2 into a gas-lift well. The lift-gas supply line 2 includes a gasmeasuring device, such as an orifice meter (not shown). The lift-gasenters the annulus between the well casing 3 and the production tubing4. The well casing may be plugged at the bottom of the well with abridge plug 5. The production tubing 4 is anchored near the bottom ofthe well with a packer 6, which also serves to isolate the annulusbetween the well casing 3 and production tubing 4 from the producingzone, identified by perforations 7 in the casing. Reservoir fluids fromthe formation enter the casing through perforations 7, and are lifted upthe production tubing. There are several gas-lift valves 8 along thelength of the production tubing at multiple depths. A dummy valve 9 mayalso be included near the bottom of the production tubing but above thepacker 6. Fluids produced from the well exit from the well through aproduced fluid line 10 and flow to a produced fluids collection facility11. To start up the well, lift gas is injected into the annulus betweenthe casing and the production tubing, to provide lift for fluids betweeneach gas-lift valve and the surface. The gas-lift valves are initiallyopen. Once some flow is established, the gas-lift valves closeautomatically as the pressure in the annulus between the casing and theproduction tubing declines, until only the bottom gas-lift valve is leftopen.

In order to apply the method of the subject invention to a well, it isnecessary to know certain basic information about the well, such as thevolume of the annulus between the casing and the production tubing perunit of depth, the approximate location/depth of the lift-gas valvesincluded in the production tubing, and the flow rate for lift-gassupplied to the well system. From this information, the time requiredfor the injected fluid or tracer to return to the surface in a producedfluid, the expected response time, may be calculated.

The following is a detailed description of the method of the subjectinvention, with specific reference to FIG. 1. As shown in FIG. 1, aninjected fluid supply source 12 is connected to the lift-gas supply line2 by an injected fluid supply line 13, which contains at least one valve14. The injected fluid is and/or generates one or more tracers in one ormore reservoir fluids. A quantity of an injected fluid, which issufficient to provide a tracer in a quantity that will be detectable ina produced fluid recovered from the well, is injected as a pulse intothe lift-gas supply line 2 near the well. The lift-gas, containinginjected fluid, travels down the annulus between the well casing 3 andthe production tubing 4, entering any openings along the productiontubing. The injected fluid enters the production tubing with thelift-gas and comingles with the reservoir fluids. Some injected fluidsgenerate tracers when combined with reservoir fluids. An analyzer 15 isattached to the produced fluid line 10. A separator (not shown) may beassociated with the analyzer to segregate a particular produced fluidfor analysis. The analyzer 15 is used to detect the presence and/ormonitor the proportion of a tracer, which may be the injected fluidand/or a tracer generated by the injected fluid, in a produced fluid asa function of time. The analyzer may be connected to a recorder (notshown) to provide a graphical representation of the data. By knowing thevolume of the annulus between the production tubing and the well casingper unit of depth, and the flow rate of the lift-gas supplied to thewell, it is possible to determine the point of entry of the injectedfluid along the depth of the production tubing from the tracer data. Bydetermining the point of entry of the injected fluid into the productionfluid, the point of entry of the lift-gas into the production tubing mayalso be determined. There may be several points of entry of lift-gasalong the production tubing. Multiple entry points indicate opengas-lift valves and/or tubing leaks. By knowing the number andapproximate depths of the various gas-lift valves, it is possible todetermine which valves are open, which valves are closed, and whetherthere are any leaks in the production tubing.

Any of a variety of injected fluids may be used in the practice of thesubject invention. The injected fluid must be injectable in a sufficientquantity and over a short time period such that it, or any tracer itgenerates may be detected in and distinguished from fluids produced fromthe well. Gases are preferred for their ease of use and miscibility withthe lift-gas. Examples of such gases include carbon dioxide, nitrogen,nitric oxide, ammonia, hydrogen, oxygen, sulfur dioxide, and halogenatedhydrocarbons such as freon. Each of these gases may be used as tracers,and several will also generate additional tracers when in contact withreservoir fluids. For example, carbon dioxide, when in contact withwater, generates disassociated carbonic acid ions, either of which mayserve as a tracer in the produced water. Radioactively tagged versionsof such materials may also be used.

Since the injected fluid, or any tracer it generates, is detectable in aproduced fluid, it is also possible to indirectly determine the presenceof the tracer in a produced fluid. Each produced fluid has one or morecomponents, for example, the produced gas contains both gas from theformation and injected lift-gas. The lift-gas itself contains methaneand a variety of other component gases. The proportion of tracer presentin the produced fluid is indirectly determined by monitoring a reductionin the proportion of one or more components present in the producedfluid, where the reduction has occurred due to the presence of a tracerin the produced fluid. In effect, a reduction in the proportion of acomponent present in a produced fluid serves as a tracer.

It is preferable to inject the injected fluid as a pulse, or largequantity over a short period of time. This allows the use of peakanalysis for the tracer to determine the entry points of the injectedfluid, and thereby the lift-gas, into the production tubing. As analternative, some minimum constant concentration of injected fluid isinjected continuously, and frontal analysis is used to determine entrypoints into the production tubing. However, the continuous injectionmethod could require a greater quantity of injected fluid than the pulseinjection method.

Any of a variety of conventional analyzers may be used to detect and/ormonitor the tracer in the produced fluids. The analyzer is selectedbased on the tracer to be detected and the produced fluid to beanalyzed. Some analyzers are sensitive to the presence of certainforeign materials, for example, water in a gas stream analyzer.Consequently, it may be desirable to include a small separator toselectively segregate a particular phase or portion from the producedfluids, such as a dry gas stream. The analyzer is calibrated to thefluid analyzed, and it may be necessary to subtract off a backgroundlevel of tracer in the produced fluid. For example, where carbon dioxideis used as the tracer, the produced fluids may already include someminimal quantity of carbon dioxide. It is desirable to connect theanalyzer to a recorder, which may provide a graphical representation ofthe data.

In a preferred method of the subject invention, the injected fluid iscarbon dioxide, and carbon dioxide serves as the tracer. The carbondioxide is supplied to the lift-gas as a condensed fluid, which allowsinjection of a quantity sufficient to be detected in produced fluidsrecovered from the well in a short amount of time. The carbon dioxide issupplied from a gas cylinder obtained from a gas supply manufacturer.Characteristics of the well are used to calculate an estimated time forthe injected carbon dioxide to return in the produced fluids. Thepresence of the carbon dioxide is preferably detected in the producedgas from the well. A slip stream of produced gas is obtained byinstalling a small separator on the produced fluid line. The gasobtained from the separator is directed to a carbon dioxide analyzer.The concentration of carbon dioxide in the produced gas is determined asa function of time and plotted by an analog recorder. By knowing thelift-gas supply rate and the volume of the annulus between the wellcasing and the production tubing per unit of depth, it is possible tocorrelate peaks of carbon dioxide in the produced gas with points ofentry of the carbon dioxide into the production tubing, therebydetermining the points of entry of the lift-gas into the productiontubing. By knowing the number and depths of the gas-lift valves alongthe production tubing, it is possible to determine whether those valvesare open or closed, and whether there are any leaks in the productiontubing.

A number of alternate methods may be used to obtain the same results.For example, where the injected fluid or tracer is soluble in oil, suchas carbon dioxide, it is also possible to detect the tracer in theproduced oil. As another alternative, where the injected fluid or traceris soluble in water, it is possible to detect the tracer in the producedwater. For example, carbon dioxide, when in contact with water,generates disassociated ions of carbonic acid, which may be readilydetected in the produced water by pH measurements. The carbon dioxideboth serves as a tracer itself, in both the produced gas and theproduced oil, and generates two tracers (hydrogen carbonate ion andhydrogen ion) in the produced water. Where carbon dioxide is employed asthe injected fluid, and hydrogen ion is employed as the tracer in theproduced water, the presence of the tracer may be identified simplythrough pH measurements.

Ammonia is another gas that may be used as an injected fluid andmonitored in the produced gas or oil. When in contact with water,ammonia generates disassociated ions of ammonium hydroxide (ammoniumions and hydroxide ions). The hydroxide ions are readily detected in theproduced water by pH measurements. When the hydrogen ion concentration,or pH, indicates the proportion of tracer present, the pH of theproduced water is measured as a function of time and the peaks (orvalleys) are correlated with entry points along the tubing. Where pH ismonitored as an indicator of the tracer, it may be possible to simplyinclude a pH detector in a slipstream on the produced fluid line.Detectors for other tracers, such as ammonium ions, could be similarlyemployed.

It is also possible to monitor for a parameter that serves as anindirect indication of the presence of a tracer. For example, ifnitrogen is used as a tracer, rather than monitoring the produced gasfor the presence of nitrogen, the produced gas could be monitored forBTU content as a function of time. The BTU content of the produced gaswould decline as the proportion of nitrogen present in the produced gasincreased. Thus, BTU content would serve as an indirect indication ofthe proportion of the tracer present in the produced gas as a functionof time. A BTU measuring device could be installed on a slip stream onthe produced fluid line. Again, indications of the portion of the tracerpresent in a produced fluid as a function of time may be used asdescribed above to determine whether valves installed in the productiontubing are open or closed, and whether there are leaks in the productiontubing.

The method may also be used to quantify the amount of gas entering at agiven point along the production tubing. Where a graphicalrepresentation of peaks, indicating the proportion of tracer present asa function of time is obtained, the fraction of tracer entering at eachentry point along the production tubing may be determined by integratingthe area under each peak and ratioing the area of each peak to the sumof the areas under all of the peaks. By knowing the amount of lift gassupplied to the well, it is possible to quantify the amount of lift gasentering the production tubing at each point of entry.

The method may also be used to detect casing leaks, where fluids fromthe well are leaking from the casing. By performing a material balanceon the injected fluid and/or tracer entering and exiting from the well,it is possible to determine whether some of the injected fluid or traceris lost, indicating a casing leak. If a casing leak is located above thebottom gas-lift valve, a longer than expected response time alsoindicates a casing leak.

In order to present a clear understanding of the present invention, themethod will now be described in more detail by means of examples. Aswill be understood, prior to applying the method to a particulargas-lift well, it is desirable to obtain specific information about thewell, such as the volume of the annulus between the production tubingand the well casing per unit of depth, the approximate location ofgas-lift valves along the production tubing, and the flow rate of thelift-gas to the well. Also, it may be desirable to perform certainpreliminary calculations, such as estimating an expected response time,the amount of time that will elapse between injection of the injectedfluid into the lift-gas line and the first exit of some the tracer influids produced from the well. Such calculations are useful in planningthe troubleshooting procedure for a well.

Various modifications of the present invention will become apparent tothose skilled in the art from the foregoing description and thefollowing examples. Such modifications are intended to fall within thescope of the appended claims.

EXAMPLE 1

The method of the subject invention was applied to a gas-lift well inLouisiana. The flow rate of the lift-gas entering the well was meteredby a standard orifice plate method. The lift-gas at the well wasinjected at a pressure of 1,150 psi and supplied at a rate of 345,000standard cubic feet per day (SCF/D). (An accurate measurement of thelift-gas flow rate is important in these calculations.) The well wasknown to have an annulus volume of 1 cubic foot per 5.433 linear feet.At average casing pressure and temperature, this resulted in a linearvelocity for the lift-gas of about 986 feet per hour in the annulus.Because the cross-sectional area of the casing-tubing annulus was muchgreater than the cross-sectional area of the production tubing, andbecause the production tubing operated at lower pressure, the velocityof the gas in the annulus was much less than the velocity of the gasreturning in the production tubing. Consequently, the amount of timerequired for lift-gas entering the well to return to the surface, theexpected response time, was assumed to be approximately equal to theamount of time required for the lift gas to travel down the annulus andenter into the production tubing. (This assumption would be valid formost wells; however, a correction for travel in the production tubingcould be made.) The well was known to have five gas-lift valves alongthe production tubing, with the bottom three valves located atapproximately 3,592 feet, 4,100 feet, and 4,593 feet. Assuming only thebottom gas-lift valve was open on the production tubing, and that therewere no leaks in the production tubing, the expected response time wascalculated as 280 minutes.

Carbon dioxide was selected as the injected fluid/tracer due to its lowcost and ease of use. In order to inject the gas over a minimum amountof time into the lift-gas line operating at 1,150 psi, the carbondioxide was supplied as a condensed gas. A gas cylinder containing about60 pounds of carbon dioxide under a nitrogen blanket was obtained from agas supply manufacturer. The gas cylinder was connected to the lift-gasline with some stainless steel tubing.

A slipstream of produced gas was separated from the produced fluids linethrough use of a three-stage, low pressure separator, similar to thosecommonly available in the field. An infrared (IR) spectrometer was usedto continuously measure and digitally record the concentration of carbondioxide present in the produced gas. The IR analyzer was connected to ananalog chart recorder, to provide a graphical representation of theconcentration of carbon dioxide present in the lift-gas as a function oftime.

To start the test, the valve on the gas cylinder was opened, allowingthe carbon dioxide to enter the lift-gas line. It was expected thatcarbon dioxide would be detected by the analyzer in the produced gasslipstream about 280 minutes later. However, after only 221 minutes, apeak representing entry of carbon dioxide into the production tubing wasidentified by the analyzer. Subsequent peaks occurred at about 265minutes and 341 minutes. These results indicated that more than onegas-lift valve was open on the production tubing and/or that theproduction tubing had a leak. A graphical representation of theseresults is shown in FIG. 2.

Since it was clear from this analyzer data that there were three entrypoints into the production tubing, it was concluded that furtheranalysis of the data would be required. The first entry point wascalculated by multiplying the linear velocity of the lift-gas (986 feetper hour) by the acual response time (221/60=3.68 hours), whichindicated entry of the lift-gas into the production tubing at about3,631 feet. The gas-lift valve closest to this entry point is located ata depth of about 3,592 feet. Since the well was new, and a tubing leakwas therefore unlikely, it was concluded that entry point occurred atthe closest gas-lift valve, which was only 39 feet away. (Data on thelocation of gas-lift valves along the production tubing are not alwaysstrictly accurate.)

Above the first point of entry, the entire gas flow rate to the wellcontributed to the linear velocity of gas in the annulus. However, oncesome gas entered the production tubing, that linear velocity was reducedbelow the point of entry by the fraction of total gas that entered theproduction tubing. The relative contribution of each point of gas entrywas estimated from the relative size of each carbon dioxide peak. Byintegration, it was determined that 27 percent of the lift-gas enteredat the first entry point, 42 percent at the second entry point, and 31percent at the third entry point, as noted on FIG. 2. Thus, the linearvelocity of gas traveling between the first entry point and the secondentry point was calculated to be 720 feet per hour (73 percent of 986feet per hour).

The second entry point was calculated by adding the product of this flowrate to the second entry point (720 feet per hour) and the incrementaltime of travel between the first and second entry points (265-221=44minutes, or 0.73 hours) to the depth of the first entry point(3,631+527=4,158 feet). Since this depth was within 58 feet of agas-lift valve located at about 4,100 feet, it was assumed that lift-gaswas entering the production tubing at this gas-lift valve. The thirdentry point was similarly calculated to be 4,545 feet. The calculatedvalue for the third point of entry (4,545 feet) was 48 feet from thebottom gas-lift valve located at about 4,593 feet. It was concluded thatlift-gas was entering the production tubing at this bottom valve.

The results of these calculations were summarized in Table 1. Calculatedresponse or entry point depths were compared with assumed depths of thebottom three gas-lift valves, and the difference in depth wascalculated. This difference was also expressed as a percentage of theassumed valve depth. The percent difference showed an accuracy of ±1.5percent, excellent accuracy by oil field standards.

                  TABLE 1                                                         ______________________________________                                        GAS-LIFT ANALYSIS TEST RESULTS                                                CALCULATED               DEPTH                                                RESPONSE   INSTALLED     DIFFER-   PERCENT                                    DEPTH      VALVE DEPTH   ENCE      DIFFER-                                    (in feet)  (in feet)     (in feet) ENCE                                       ______________________________________                                        3,631      3,592         +39       +1.1                                       4,158      4,100         +58       +1.4                                       4,545      4,593         -48       -1.0                                       ______________________________________                                    

The amount of lift-gas entering each valve was calculated from thelift-gas supply rate and the fraction of lift-gas entering at each entrypoint. The first valve allowed 27 percent of the lift-gas to enter theproduction tubing, or 93,150 SCF/D (27 percent of 345,000 SCF/D). Theamounts for the second and third valves were similarly calculated to be144,900 SCF/D and 106,950 SCF/D, respectively.

Since it was expected that only one peak would be identified, theanalyzer may have been set for too coarse of a time interval to allowoptimum accuracy. Also, less carbon dioxide could have been used. If thetest had been repeated, better peak definition and better correlationbetween gas entry points and actual gas-lift valve depths would havebeen obtained. Alternative calculation methods could have been used. Forexample, the flow rate of lift-gas entering a gas-lift valve (orproduction tubing leak) may be calculated from the ratio of the gasvelocities above and below each point of entry.

EXAMPLE 2

In this hypothetical example, the characteristics of the well aresimilar to the well described in Example 1. The well has an annulusvolume of 1 cubic foot per 5.433 linear feet, and the lift-gas flow rateis determined to be 350,000 SCF/D. The linear velocity of the lift-gasat average well conditions is 1,000 feet per hour. Carbon dioxide isused as the tracer and monitored in the produced gas, as in Example 1.The bottom active gas-lift valve is located at approximately 5,000 feet.Assuming no leaks in the casing, the tracer will reach the open gas-liftvalve and return to the surface in five hours.

However, there is no peak after five hours, and the peak occurs onlyafter six hours, one hour later than expected. As in Example 1, thetotal response time for the tracer is the time it takes for the tracerto reach the furthest point of entry on the production tubing. Since theactual response time is greater than the expected response time, thisindicates that the velocity is reduced due to a leak from the casingabove the bottom open gas-lift valve. It is determined, by quantifyingthe carbon dioxide in the produced gas, that only 75 percent of thecarbon dioxide returns. Consequently, 25 percent of the tracer is lostto the leak.

The linear velocity of the lift-gas above the leak is known to be 1,000feet/hour. If 25 percent of the lift-gas is lost to a leak, then thelinear velocity of the lift-gas traveling between that leak and thebottom gas-lift valve 75 percent of that velocity, or 750 feet/hour.Knowing these two linear velocities, it is possible to calculate thedepth of the leak as follows: ##EQU1## Where: ^(t) r=Response time forthe tracer.

d_(bv) =Depth of bottom gas-lift valve.

x=Fraction of the depth to the bottom gas-lift valve at which a leakexists.

v₁ =Gas velocity above casing leak.

v₂ =Gas velocity below casing leak.

It is known that the response time for the tracer is six hours, thedepth of the bottom gas-lift is 5,000 feet, the gas velocity above thecasing leak is 1,000 feet/hour, and that the velocity of the gas belowthe casing leak is 750 feet/hour. It is calculated that x=0.40, and thatthe depth of the casing leak is 40 percent of the depth to the bottomgas-lift valve, resulting in a casing leak depth of 2,000 feet.

In the alternative, if a casing leak is below the open gas-lift valve, apartial loss of the tracer will again occur; however, the remainingtracer will return at the expected response time. This is because thelinear velocity of the lift-gas will be the calculated value based onthe supply rate when the lift-gas enters the open gas-lift valve.Consequently, it is not possible to determine the depth of a casing leaklocated at or below the bottom gas-lift valve, only that the casing leakis at a depth equal to or greater than the open gas-lift valve depth.The leak may be quantified through material balance calculations on thetracer.

EXAMPLE 3

In this hypothetical example, the characteristics of the well aresimilar to the well described in the examples above. The linear velocityof the lift-gas at average well conditions is 1,000 feet/hour, and thebottom active gas-lift valve is located at a depth of 5,000 feet. Carbondioxide is used as the tracer and monitored in the produced gas, as inExample 1. The expected response time for the well, the elapsed time atwhich injected tracer should return to the surface, is calculated to befive hours. The presence of a tracer is detected in a well-defined peakat about five hours after the tracer entered the well. It is concludedthat the well's gas-lift system is operating properly, with liftoccurring solely off the bottom active gas lift valve. Since most of theother wells in the field are similarly designed, most of the wells havethe same expected response time. Consequently, the other wells aresimilarly checked to see whether they are operating properly.

What is claimed is:
 1. A method for determining a point of entry of alift-gas into a production tubing in a gas-lift well in a subterraneanreservoir comprising:injecting into the lift-gas supplied to thegas-lift well a quantity of an injected fluid, which is at least one of(1) a tracer (2) a tracer generator, in a quantity which is sufficientto be detected in a produced fluid recovered from the well; monitoring aproportion of the tracer in the produced fluid as a function of time;determining the point of entry of the injected fluid into the productiontubing, thereby determining the point of entry of the lift-gas into theproduction tubing.
 2. The method of claim 1 wherein the point of entryof the lift-gas into the production tubing is correlated with locationsof gas-lift valves in the production tubing to determine which of thevalves is open.
 3. The method of claim 1 wherein the point of entry ofthe lift-gas into the production tubing is correlated with locations ofgas-lift valves in the production tubing to determine which of thevalves is closed.
 4. The method of claim 1 wherein the point of entry ofthe lift-gas into the production tubing is correlated with locations ofgas-lift valves in the production tubing to determine which of thevalves are open, and which of the valves are closed.
 5. The method ofclaim 1 wherein the point of entry of the lift-gas into the productiontubing is correlated with locations of gas-lift valves in the productiontubing to identify a leak in the production tubing.
 6. The method ofclaim 1 wherein the proportion of the tracer in the produced fluid as afunction of time is used to determine the fraction of tracer, andthereby the fraction of lift-gas entering the production tubing at eachpoint of entry.
 7. A method for determining a point of entry of alift-gas into a production tubing in a gas-lift well in a subterraneanreservoir comprising:injecting into lift-gas supplied to the gas-liftwell a quantity of an injected fluid, which is or generates a tracer,the tracer reducing the amount of a component in a produced fluidrecovered from the well in a quantity which is sufficient to be detectedin the produced fluid; monitoring the reduction in a proportion of thecomponent in the produced fluid as a function of time; determining thepoint of entry of the injected fluid into the production tubing, therebydetermining the point of entry of the lift-gas into the productiontubing.
 8. The method of claim 7 wherein the point of entry of thelift-gas into the production tubing is correlated with locations ofgas-lift valves in the production tubing to determine which of thevalves is open.
 9. The method of claim 7 wherein the point of entry ofthe lift-gas into the production tubing is correlated with locations ofgas-lift valves in the production tubing to determine which of thevalves is closed.
 10. The method of claim 7 wherein the point of entryof the lift-gas into the production tubing is correlated with locationsof gas-lift valves in the production tubing to determine which of thevalves are open, and which of the valves are closed.
 11. The method ofclaim 7 wherein the point of entry of the lift-gas into the productiontubing is correlated with locations of gas-lift valves in the productiontubing to identify a leak in the production tubing.
 12. The method ofclaim 7 wherein the reduction in the proportion of a component in theproduced fluid as a function of time is used to determine the fractionof tracer, and thereby the fraction of lift-gas entering the productiontubing at each point of entry.
 13. A method for determining the point ofentry of a lift-gas into a production tubing in a gas-lift well in asubterranean reservoir comprising:injecting into the lift-gas a quantityof carbon dioxide which is sufficient to be detected in produced gasrecovered from the well; monitoring the concentration of the carbondioxide in the produced gas as a function of time; determining the pointof entry of the carbon dioxide into the production tubing, therebydetermining the point of entry of the lift-gas into the productiontubing.
 14. A method for determining the point of entry of a lift-gasinto a production tubing in a gas-lift well in a subterranean reservoircomprising:injecting into the lift-gas a quantity of carbon dioxide,which generates carbonic acid ions in a quantity that is sufficient tobe detected in produced water recovered from the well; monitoring theconcentration of the carbonic acid ions in the produced water a functionof time; determining the point of entry of the carbon dioxide into theproduction tubing, thereby determining the point of entry of thelift-gas into the production tubing.
 15. A method for determining thepoint of entry of a lift-gas into a production tubing in a gas-lift wellin a subterranean reservoir comprising:injecting into the lift-gas aquantity of carbon dioxide which is sufficient to be detected inproduced gas recovered from the well; monitoring a reduction in theconcentration of a component in the produced gas as a function of time;determining the point of entry of the carbon dioxide into the productiontubing, thereby determining the point of entry of the lift-gas into theproduction tubing.